Gas-cooled reactors



Dec. 27, 1966 A. BELL GAs-cooLED REAcToRs 2 Sheets-Sheet l Filed March30, 1964 INVENTOR. ALAN BELL ATTORNE Y Dec. 27, 1966 A. BELL 3,294070GAS COOLED REACTORS Filed March 50, 1964 2 Sheets-Sheet 2 INVENTOR.

ALAN BELL NIU lma( M mfom@ ATTORNEY United States Patent O 3,294,070GAS-CLED REACTORS Alan Bell, Coolrham, England, assignor to FosterWheeler gororation, New Yorlr, N.Y., a corporation of New Filed Mar. 30,1964, Ser. No. 355,797

Claims priority, application Great Britain, Apr. 8, 1963,

Claims. (Cl. 122-32) This invention relates to a once-through forcedcirculation tubular steam generator, and in particular to a finalevaporation and superheating section of a once-through generator. Theterms water and steam are used herein to denote any suitable liquid andits vapour.

Such steam generators generally include preheating, evaporating andsuperheating sections connected in series. The steam and water mixtureproduced in the evaporator may be very heterogeneous and there is thenthe danger that the mixture may pass through the superheater without'complete evaporation of its water content. It is to the solution ofthat problem that the present invention is directed.

In a steam generator in accordance with the invention, the steam andwater mixture produced in a tubular evaporator is passed to .a tubularsuperheater in which it is subjected to high speed turbulent ow in orderto improve the homogeneity of the mixture before being subjected tolower speed ow for evaporation of the dispersed water and the finallydesired superheating.

Preferably the high speed turbulent flow is effected by passing thewhole of the mixture flowing from the evaporator through a single tube.This can be important because, if the mixture from the evaporator isoffered alternative paths, there may be segregation of 'water and steamfrom the mixture With the result that the streams passing over the saidpaths will be of different composition.

It is important that suflicient heat transfer surface is provided toallow evaporation lof any water in the steam to occur. This :can beconveniently achieved by providing as long as ow path for thesteam-water mixture as possible. To this end, the mixture can be iiowedat high speed from one end to the other of the superheating section andthen back again before being flowed at the lower speed Vin the originaldirection to the superheated steam outlet. In the preferred form of theinvention, the mixture is flowed iii-st through a single tube ofrelatively large diameter disposed along the axis of the superheatingsection, then in the opposite direction through two tubes the aggregatecross-section of which is approximately equal to the cross-section ofthe single tube and which are connected thereto in parallel and finallyin the original direction through a number of relatively small diametertubes surrounding the other tubes and connected in parallel with theintermediate tubes.

The rate of heat transfer in the nal stage of ow through the superheatersection should be as high as possible and this can be achieved by makingthat flow turbulent, the turbulence here being thus for a differentpurpose from that for which turbulence is created in the initial stageof superheating.

An example of the invention will now be described with reference to theaccompanying drawings in which:

FIGURES 1 and 1A are sectional elevation views of an element inaccordance with the invention;

FIGURE 2 and FIGURE 3 are enlarged cross-sections .taken on the linesII-II and III-III respectively of FIG. 1; and

FIGURE 4 is an elevation section view of a steam generator using anelement in accordance with the invention.

The element 12 shown in FIGS. 1 and 1A may, in practice, have an overalllength of about 25 and may be arranged vertically side-by-side as shownin FIG. 4 with a large number of identical elements within a shell orcasing 14 to form a steam generator 16. Each element comprisesrapreheating section 118, a steam generating section 20, and a steamsuperheating section 22 connected together for series flow of fluidthrough the element.

The preheating and steam generating sections may be identical andcomprise (FIGS. 1 and 2) nineteen substantially straight tubes 24connected at their ends to tube plates 26 to form a cluster 28 of tubes.The two sections are connected together in series by welding anintermediate header member 30 between adjacent tube plates of eachsection. To the tube plate at the other end of the preheating section isattached a header 32 for receiving the water to be evaporated.

The -superheater section 22 comprises a cluster of nineteen tubes (FIG.3) generally designated 34 c-onnected at their ends to tube plates 36and 38 respectively. The section is connected in series with the steamgenerating section 26 through an intermediate header 46 in which isprovided a funnel shaped baille 44 which serves to direct the mixtureyfrom the steam generating section into the central tube 46 of the tubecluster. The fluid leaving this central tube is deflected by ahemispherieal member 48 attached to the tube plate 38 into two tubes 50adjacent the central tu'be 46. The iiuid flows through these two tubesin a direction opposite to the flow through the central tube 46 and atthe outlet thereof is deected by .the bafiie 44 into the remainingsixteen tubes 52 of the cluster 18 through which it flows in the samedirection as through the central tube 46. The uid finally leaves thecluster through a header 54.

In the arrangement described above, the steam and water mixture producedin the nineteen parallel tubes of the evaporator section is on leavingthat section caused to flow through the single tube 46 of thesuperheater section and then through the two adjacent tubes Sti. Thespeed of the mixture owing through that part of the superheater sectionconstituted by the tubes 46 and 50 is thus considerably higher thanthrough the evaporator section and by adjusting the rate at which wateris supplied to the element, the `speed of the mixture through the tubes46 and 5() may be made sufficient to cause the ow through them to behighly turbulent. This improves the homogeneity of the mixture and thehomogeneity is still further improved by the change in direction twiceimposed on the ow through the superheater section. On leaving the twotubes 50, the mixture flows through the remaining sixteen tubes 52 ofthe superheater section where the velocity of flow is reduced to thatthrough the evaporator section.

The tubes of each section may all be of the same diameter. Preferably,however, the central tube 46 of the tube cluster 34 through which themixture leaving the evaporating section is flowed is of a diameter suchthat its cross-sectional area is approximately equal to the sum of theeross-sectional areas of the two tubes 50. This has the advantage ofenabling each tube cluster to be composed of nineteen tubes, evenly andclosely pitched on their end tube plates. In a typical example of theelement described above, the inside diameter of the central tube 46 is1" and that of the remaining tubes of the superheater section is v1/2.

It will be appreciated that the tubes of the heat exchange elements neednot be exactly arranged as described and shown here. For example, thetube arrangement described in the co-pending application Serial No.276,048, filed April 23, 1963, now Patent No. 3,195,517 may be used inthe preheating and steam generating sections. Also the single tube which.forms the rst part of the flow path through the superheating sectionneed not be the central tube of the cluster, although this arrangementis the easiest to construct.

As shown in FIG. 1, all the tubes except the central tube 46 areprovided with means 56 in the form of iluted bars for increasing theturbulence of the fluid flow through them. This improves the heat inputinto each tube and enables the overall length of the heat exchangeelement to be kept short.

Although not illustrated here, the vapour generating section 20 may beconstructed as shown in our copending application Serial No. 276,048,led April, 1963, now Patent No. 3,195,517 with a central water downcomertube.

Any number of the heat exchange elements may be arranged in any desiredmanner within a shell or casing to form a steam generator. An example isshown in FIG. 4. In the unit of FIG. 4, a plurality of the tube systemsof FIGS. 1-3 extend between lower and upper collecting `headers 58 and60, the small headers 32 and 54 being joined to the collecting headersby lower and upper risers 62 and 64. A rectangular shell 66 surroundsthe tube systems, provided with a gas inlet 68 at the top thereof. Thegas tlow is essentially longitudinal of the elements, i.e., parallel tothe tube axes. As shown the headers 32 and 54 are staggered for thispurpose, but in addition, the headers and spacer plates 70 (to bedescribed) cause a partial cross ilow.

A typical generator may consist of a plurality of units bolted together,each unit being shop assembled. One unit may contain nineteen tubes. Thegenerator then with a length of about 25 feet will provide a heatingsurface comparable to that of a conventional unit a hundred feet plus.In gas cooled nuclear stations, the saving in pressure vessel volume andthickness should be apparent. The size of the casing in any given casecan be kept to a minimum by arranging the elements close together sothat there is as little lost casing space as possible. This is assistedby having the tubes arranged on the tube plate as shown in FIGS. 2 and3, by using intermediate headers 30, 40 which have a circular orhexagonal cross-section and by springing or otherwise deforming thetubes of each section so as to achieve uniform distribution of the tubesof the elements over the cross-section of the casing. This may be doneas shown in FIGS. 1, 1A and 2 by spacer plates 70 whose detailedconstruction and method of assembly on each tube cluster is described inour copending Application Serial No. 243,926, tiled December 1l, 1962,now Patent No. 3,227,142.

Although the invention has been described with relation to specificembodiments, it is to be understood that many variations are within thespirit and scope of the invention as claimed.

What is claimed is:

1. In a forced circulation boiler having a tube system including inseries preheating, evaporating and superheating sections, theimprovement comprising in said superheating section a plurality of tubesin a cluster including a center tube and rst and second sets of tubessubstantially parallel to, coextensive with and removed from said centertube on opposite sides thereof, the center tube having a cross-sectionalarea of flow substantially less than the cross-sectional area of ilow inthe evaporating section, -said cluster of tubes delining a circulararray encompassing the center tube, said rst set of tubes being twotubes on opposite sides of the center tube, said crosssectional areas ofthe center tube and first set of tubes being substantially equal, meansfor directing the llow from the evaporating section to said center tube,second means for directing the ilow from the center tube to said rst setof superheating section tubes, and third means -for directing the ilowtherefrom to the second of said sets of tubes so that the ow leaves thesuperheating section at lan end removed from the evaporating section.

2. In a forced circulation boiler, a tube system comprising a pluralitylof tube elements,

each element including in series a preheating section, an

evaporating section, and a superheating section, the sections beingaxially aligned,

each section comprising a center tube and inner and outer `circulararrays of tubes, the tubes being substantially parallel and coextensive,

inlet and outlet headers for each element communicating with thepreheating and superheating sections respectively,

intermediate headers communicating the sections with each other, thecenter tube of the superheating section including a funnel-shaped bailledisposed in the header intermediate the evaporating and superheatingsections arranged to direct the flow from the evaporating section intothe center tube, the center tube having a cross-sectional area of ilowsubstantially less than the cross-sectional area of llow in theevaporating section,

the superheating section further including a hemispherical shaped bailleto direct the flow from the center tube to two tubes adjacent and onopposite sides of the center tube,

the funnel-shaped baille further being arranged to direct the flow fromsaid two tubes to the remaining tubes of the superheating section,whereby the ow enters the outlet header at an end of the element removedfrom the inlet header.

3. A boiler according to claim 2 further including means for increasingturbulence in said remaining tubes of the superheating section.

4. A boiler according to claim 1 wherein said means for directing the owfrom the evaporator section to said center tube includes a funnel-shapedbaille.

5. In a forced circulation boiler having a tube system including inseries preheating, evaporating and superheating sections, theimprovement comprising in said superheater section a plurality of tubesin a cluster including a center tube and rst and second sets of tubessubstantially parallel to, coextensive with and removed from said centertube on opposite sides thereof, the center tube having a cross-sectionalarea of ilow substantially less than the cross-sectional area of flow inthe evaporating section, means including a lfunnel-shaped battle fordirecting the ilow from the evaporating section to said center tube,second means including a hemi-spherical-shaped baille for directing theflow from the center tube to said first set of superheating sectiontubes, and third means coextensive with said funnel-shaped baille fordirecting the ow therefrom to the second of said sets of tubes so thatthe flow leaves the superheating section at an end removed from theevaporating section.

References Cited by the Examiner UNITED STATES PATENTS 2,083,679 6/1937Adams 165-158 X FOREIGN PATENTS 1,205,952 8/ 1959 France. 1,231,3'11 4/1960 France. 1,250,438 12/:1960 France.

909,537 10/ 1962 Great Britain.

KENNETH W. SPRAGUE, Primary Examiner.

1. IN A FORCED CIRCULATION BOILER HAVING A TUBE SYSTEM INCLUDING INSERIES PREHEATING, EVAPORATING AND SUPERHEATING SECTIONS, THEIMPROVEMENT COMPRISING IN SAID SUPERHEATING SECTION A PLURALITY OF TUBESIN A CLUSTER INCLUDING A CENTER TUBE AND FIRST AN SECOND SETS OF TUBESSUBSTANTIALLY PARALLEL TO, COEXTENSIVE WITH AND REMOVED FROM SAID CENTERTUBE ON OPPOSITE SIDES THEREOF, THE CENTER TUBE HAVING A CROSS-SECTONALAREA OF FLOW SUBSTANTIALLY LESS THAN THE CROSS-SECTIONAL OF FLOW IN THEEVAPORATING SECTION, SAID CLUSTER OF TUBE DEFINING A CIRCULAR ARRAYENCOMPASSING THE CENTER TUBE, SAID FIRST SET OF TUBES BEING TWO TUBES ONOPPOSITE SIDES OF THE CENTER TUBE, SAID CROSSSECTIONAL AREAS OF THECENTER TUBE AND FIRST SET OF TUBES BEING SUBSTANTIALLY EQUAL, MEANS FORDIRECTING THE FLOW FROM THE EVAPORATING SECTION TO SAID CENTER TUBE,SECOND MEANS FOR DIRECTING THE FLOW FROM THE CENTER TUBE TO SAID FIRSTSET TO SUPERHEATING SECTION TUBES, AND THIRD MEANS FOR DIRECTING THEFLOW THEREFROM TO THE SECOND OF SAID SETS OF TUBES SO THAT THE FLOWLEAVES THE SUPERHEATING SECTION AT AN END REMOVED FROM THE EVAPORATINGSECTION.